1. Field of the Invention
The present invention relates to mirrors, and more particularly, to a protective coating for the silver layer of a silvered glass mirror to protect the silver layer from degradation.
2. Description of the Prior Art
Most commercial mirrors are silvered glass composite structures wherein a thin layer of silver is deposited on the surface of a glass substrate to reflect light. Silver is used because its reflectively is significantly greater than other metals. A common commercial process for fabricating such mirrors is known as a wet chemical electroless process, wherein a thin layer of chemically reduced silver is precipitated onto a sensitized glass surface. In order to protect the silver layer from damage and degradation, a protective coating is applied to its exposed surface. A common protective coat is comprised of a copper layer deposited over the silver layer with a thick enamel paint spread over the copper layer. The copper layer interposed between the silver and paint layers enhances adhesion of the paint to the silver. A typical composite silvered glass mirror structure is shown in FIG. 1.
Silvered glass mirrors structured as described above have been used indoors without significant problems for many years. The more recent interest in use of mirrors for solar collector applications, such as heliostats, has resulted in more test installations of mirrors outdoors. While such tests have indicated economic feasibility of mirrored solar collector concepts from an energy standpoint, they unfortunately also revealed that conventional silvered glass mirrors do not stand up very well in outdoor environments. In fact, the reflective properties of most conventional mirrors undergo substantial degradation within several months to several years in outdoor environments, which is a relatively short time when a useful life of 20 years is generally considered to be minimum design requirements for economical solar collector installations.
The exact optical and mechanical degradation mechanisms of this physiochemical corrosion process are not well understood. Environmental and electrochemical test have shown that the corrosion reaction that occurs at the glass/metal interface of a mirrored surface is extremely sensitive to interfacial environment. It is believed that collection of humidity and chemicals normally found in the atmosphere in imperfections and crevices of the plated metal surface, fabrication chemicals, glass leaching, local hot spots and stresses, and photoactivation of the surface metals produces deleterious effects and causes large variances in the durability of mirrored surfaces.
Detailed surface analyses of mirrors degraded in outdoor environments have indicated several possible mechanisms for chemicaly induced changes at critical interfaces, such as at the silver/glass interface. Both iron and alkali metal ions have been found to concentrate at the silver/glass interface and are suspected of reducing the silver to glass bond strength and contributing to silver degradation when water diffuses from exposed edges or from coating flaws into the silver/glass interface. This interface is also weakened by the tendency of the glass to form a hydroxide gel layer on its surface. Such gel formation is promoted by the reduction of tin complexes (sensitizers) applied on the glass surface during the wet chemical electroless process of mirror fabrication, by water adsorbed on the glass surface from the air and by other interfacial impurities which result from subsequent mirroring steps in the fabrication process, including copper plating and enameling or painting.
In order to fabricate a better mirror structure that is capable of withstanding outdoor use for prolonged periods of time, such as 20 years or more, without degrading its reflective properties, it is necessary to protect the critical interfaces in the mirror from such destructive chemical reactions. Prior to this invention, such protection for mirrors was unknown.